It is important, in the production of certain coated papers, that the coating be applied properly and to the correct degree. For example, if the coating on thermal paper used in copying machines is too thin, poor quality of the printed image results. Excessive thickness of the coating, while not necessarily producing an unfavorable effect on image quality, needlessly increases the cost of production.
Other aspects of coatings, besides thickness, also are of interest. Often, it is necessary for a coating to be applied uniformly. In certain applications, the positioning of coating segments is important.
In the production of coated paper products, the amount of coating material applied to the base paper is expressed as coating weight. Beta gauges are the most commonly used instruments for measuring the coating weight of a coated paper. Such instruments develop coating weight indications from the degree of absorption of beta rays by the coating.
Although in fairly widespread use, beta gauges suffer from certain shortcomings. Because beta rays are absorbed by water carried in the coated paper, the degree of absorption of beta rays is a function not only of the coat weight but of the water content as well. The water content of the coated paper should be measured separately and subtracted from the overall weight. Because the quantity indicated by beta gauges is directly proportional to the weight of the coating and of the base paper, the sensitivity and accuracy of beta gauges are limited when measuring thin coatings on relatively thick base papers.